1. Field of the Invention
The present invention relates to converters. More specifically, the present invention relates to methods and apparatus for high speed digitally controlled converters for converting an input voltage to a direct current (d.c.) output voltage.
2. Background
Converters are incorporated into power supplies and are employed to transform an input voltage to a d.c. output voltage. Although advances have been made in miniaturizing electronic circuitry, power supplies have remained comparatively large.
The power density (e.g., power per unit volume) of a power supply, and thus a converter, is measured in watts per cubic inch (watts/in.sup.3). The power density of a typical converter of the prior art is approximately one watt/in.sup.3. In order for converters to adequately provide sufficient power to modern circuit designs, converter power density (e.g., capacity) must be increased while the physical size is simultaneously decreased. For example, a power density of (10 to 40) watts/in.sup.3 accompanied by a proportional reduction in volume is necessary to satisfy the requirements of current circuit designs.
Attempts have been made in the art to satisfy these requirements. Initial converter design employed a linear approach. However, the linear converter design exhibited the familiar problems of insufficient power density and excessive volume. Thereafter, the switching converter was developed. The function of the switching converter is to provide higher switching rates to increase power density and reduce converter volume.
The higher switching rate is accomplished by chopping the input voltage at a high electronic rate to provide higher frequency operation and increased power throughput. Early designs of switching converters achieved switching rates of (5-10) KHz/sec. Further, switching converters permit utilization of smaller magnetic components to reduce converter volume and also accept alternating or direct input voltages. Unfortunately, prior art switching converters having switching rates of (50-100) KHz/sec and smaller physical volumes continue to be inadequate for the power density and volume requirements of modern circuit designs.
Note that a converter having a switching rate greater than those mentioned above is not a solution to the power density and volume problems. Upon reaching a threshold frequency or switching rate, the converter tends to becomes unstable. The instability is usually caused by variations in the electrical load or inadequate phase margin and results in instability in the output voltage level. Unfortunately, control loops associated with converters known in the art have only been marginally successful in stabilizing the output voltage, particularly under no load conditions. Therefore, power supply converter design in the art continues to suffer from insufficient switching rates which results in inadequate power density, instability in the output voltage level and oversized packaging.
Thus, a need remains in the art for an improvement in conventional converter design for providing higher power density, improved stability and smaller packaging.